Renewable energy resources, such as wind and solar, are becoming more widely utilized. DC collection systems can be used to collect power from individual renewable energy resources and transmit the collected power to a utility system. Depending on the type of renewable energy resource, converters at the renewable energy sources can be AC/DC rectifiers or DC/DC inverters. Power generated from the energy sources is collected by feeders and then aggregated at a grid interface station for further power conversion and voltage transformation.
Due to small resistance and lack of inductance, when a DC fault occurs, the rate of rise of DC fault current is quite fast and the peak fault current is typically much higher than nominal current. The fast rate of rise of DC fault current creates difficulties for fault isolation, and high DC fault current may damage equipment in the DC collection system. There exist different protection strategies for DC systems. Most depend on DC circuit breakers, which are expensive.
In another conventional approach for an HVDC (high-voltage DC) system transmitting electrical power from a DC to AC network through a voltage source converter (VSC), diodes are placed in series with the VSC on either the positive or negative DC line in unipolar or bipolar configurations. When there is a fault on the DC line, the fault current is limited or interrupted. From the HVDC classic rectifier side, the polarity of the DC voltage can be reversed by the controllable rectifiers. From the VSC inverter side, the series diode blocks the fault current flow from the AC network to the fault location on the DC line. However, blocking all fault-current contributing converters with series diodes is not a desirable protection solution for a multi-terminal DC power system such as DC collection systems for renewable resources due to widespread impacts of the fault on the entire DC system. Another conventional protection device for DC collections system includes a controller for detecting and isolating faults. A probe power unit is also provided to determine whether a fault persists after fault isolation, and to enable reconnection of the isolated segment if the fault is removed. In this case, the protection device implementation is nontrivial and adds significant cost to the DC collection system.